Oil can either be burned, or it can leak
As long as the leak is small it should not cause any issues. Make sure to check the oil level daily or more frequently if on a long drive. If it is a large leak then you need to have it repaired immediately.
It means that you have a leak somewhere and the system can't keep to maintain vacuum. If the level of fluid does not drop it does not mean that you do not have a leak. It did not have time to develop.
You can find a small vacuum leak in your 2002 Toyota Camry with a diagnostic tester. The diagnostic tester can pinpoint virtually any vacuum leak.
There is not enough information to answer this question
The Camry could have a vacuum leak. If there is a vacuum leak, then there can be a whistling sound until the vacuum system is full of air.
Yes it is, you have to get it looked at ASAP.
Have you checked the Gasket that goes around the Trunk opening?
If it runs good then one of your hoses is clogged up.
You need to replace the oil seal.
You do not have many choices either replace it or if the leak is not sufficient buy one of those leak fix bottles.
Yes, it can cause check engine light to come on. Reasons are multiple - anything from air/vacuum leak, spark problems etc. to major failure.